lightning protection

[T37 Phil]

Very cool that air can be 'melted' into a conducting state.

Regarding this

If there is no current flowing in a good conductor - the field across it is zero (ohms law). If your mast is insulated by air from any source of current, it will have zero volts across it - and in an electric field, will take on the single potential of its mid point in the field.

Let's try this. Say a mast is grounded to sea water and therefore at the potential of the seawater, call it arbitrarily zero. Let's say the mast is in the uniform field you described above. Now flip a switch that disconnects the ground and isolates the mast from the seawater. What is the potential of the mast? If it changed, how does that happen? Is it instantaneous, or does it develop over time? Is a finite derivative for potential possible without an associated current? If you float the mast up 1000ft and connect an insulated perfect conductor to it back to the boat, could you use the wire like one terminal of a battery? The problem I'm having is that potential is not a state variable. Potential is the, well, potential for work to be done, in this case by an electric field. The field depends on charge distribution. So if the charge distribution in the mast is not changing, how does its potential change such that it comes to an equipotential value that is the average field value in the air along its length?

[Edit: reading further I see Bill is onto a similar line of thought.]

 

[ffaubert]

You have only 1/4 the worry that men do http://science.nasa.gov/science-news/science-at-nasa/1999/essd18jun99_1/

The stat Im having a hard time finding data on is that men which were struck were discovered with their fly down.

Maybe Im getting this mixed up..

So than a "lightning rod" is a ,....?

 

[T37 Phil]

So than a "lightning rod" is a ,....?

... only when it's flowing.

 

[ffaubert]

... only when it's flowing.

 

[walt]

Phil, you are making my head hurt (and on a Friday night..).

Here is what I think will happen.. Some experiments..

Start with a "vertical" conductive mast that is electrically connected to ground potential and then begin to raise / change the electric field in the vertical dimension.

After some time, stop raising (ie, changing) the electric field and then electrically disconnect the mast from ground..

The mast will still be at ground potential.

However if the mast is isolated / insulated from ground and then we once again start from zero potential and increase the electric field, the mast will be at the single potential of the electric field at the masts geometric mid point in the electric field. If I stop changing the electric field and there is no charge transfer from the mast, it will remain at this same potential - equal to the electric field voltage at the geometric mid point of the mast.

If I then flip the same switch and connect the mast to ground, charge will flow from the mast to ground and then once again both the mast and ground will be at the same potential. If the switch is then opened again, the mast will remain at ground potential.

I have observed another interesting thing with insulated conductors which have different vertical mid point heights (Ive seen this three times, always on a dry lake bed with a land sailer).

I have a carbon mast - mid point maybe 10 foot high and another metal structure on the boat mid point maybe 3 foot high. The two conductive structure are both isolated from each other and ground but come within about 1 cm of each other at one point.

One time while waiting for wind (back when it didnt bother me much to sail storm wind), I heard a pop - pop - pop.. this went on for maybe 5 minutes and the pops were maybe 2 seconds apart. I thought at first the mast was delaminating but on further inspection, there was a spark jumping between the two conductive structures - over and over and over.

I believe this was caused by a continually increasing electric field. If the field started off at zero and started to increase, both of these conductors took on the single voltage potential of their mid points - which were at different heights. At some point, the potential between the two conductors got high enough that a breakdown occurred and some charge was transferred between them. The amount of charge depended on the capacitance and enough charge would transfer as to reduce the potential difference enough for the ionization to collapse.

In order for the popping to continue - and it did for a long time - the electric field had to keep on increasing.

Ive only seen this on a dry lake bed.. don't know why. Ive also been shocked on my shrouds, outboard which were also insulated and I think its the same mechanism.

 

[T37 Phil]

Phil, you are making my head hurt (and on a Friday night..).
[...] Ive only seen this on a dry lake bed.. don't know why. Ive also been shocked on my shrouds, outboard which were also insulated and I think its the same mechanism.

Sorry about the headache. I suggest two long reaches and your favorite cocktail. Throw in a sunset or moon&stars if it's really bad. The short answer, I believe, is that humidity defeats static. I think your last post also confirms the need for the mast to be in a perfectly insulating medium. We're skating on thin ice with moving conductors or time-varying fields, but it seems you are still saying that the potential change change in the insulated conductor instantaneously and uniformly at every position. Is that so?

 

[Bill Roosa]

Currents and electric fields

I think all of us will agree that an electric field that has different potential (scalar electric field has different numbers in different locations)(makes a vector field BTW) in different places can cause currents to flow. A battery is a good example. The + and - terminals have air between them and so just a little current flows. If you have a dirty case top more current will flow and if you connect the two with a metal wrench a lot of current will flow. But wait, aren’t the terminals connected to each other inside the battery?!!!! Why doesn't the current flow that way? This is kinda odd so bear with me.
If I place a long metal bar vertically in the air the top of the bar is in a region of higher electrical potential than the bottom. But this potential is not the same as the one between the air gap in a battery. It is due to the earth trying to equalize the charge between itself and the ionosphere. Earth is negative and ionosphere positive (I think, could have that backward) It is a static field that EVERYTHING is immersed in. If I connect a wire to the earth the wire is also in the static field and just sits there. Each electron does not see a vector because the ones on top just see the top field; the ones on bottom just see the bottom field. You have to go OUTSIDE the system to get a vector field that will cause current to flow. Which brings us back the batteries, they don't let current flow because all the electrons are just seeing the LOCAL electric field and not the GLOBAL electric field. The "natural" electrical potential of each plate does not cause the electrons to move through the electrolyte. But take a few electrons off the negative terminal via a wire to the positive that does not go through the plates (globally bypass the plates) and the resulting unbalanced field inside the battery replaces them.

So you can have a statically varying electric field that will not produce a current flow because you have to "short" it without being part of the generating apparatus to get current to flow.

Consider the water analogy. A long pipe of water open at both ends is immersed into a lake. The pressure at the bottom is greater than the pressure at the top right? If I bring top of the pipe to the surface then water should magically start to flow because the bottom is at higher pressure right? The weight of the water keeps that from happening. Now all you have to get your head around is what is the "weight" and "substance" of an electric field. We live in a sea of electricity and there is a "weight generated" potential that we can see but not tap into.

 

[Bill Roosa]

One more thing to make your head hurt

A hydrogen atom (one proton and one electron) is about a centimeter in diameter out in space. As you bring more and more matter around it it shrinks to the very small size we "normally" see.

Why is that? The electric field is the same in both cases, right?

 

[Don S/V ILLusion]

A hydrogen atom (one proton and one electron) is about a centimeter in diameter out in space. As you bring more and more matter around it it shrinks to the very small size we "normally" see.

Why is that? The electric field is the same in both cases, right?

This reminds me of the other ponderous question Bill - what is the speed of dark?

 

[kloudie1]

Must be slower than light because Light can overtake it ! I dunno !
Kinda like the Langoleers in Stephen King.. the dark is always back there erasing the day that just happened, trying to catch the light! LOL

 

[Bill Roosa]

Speed of dark

Well Don, the speed of dark is the same as the speed of light but in the opposite direction.

More interestingly, how does dark know which way to travel since it does not have a vector field associated with it? My assumption is dark is not made of anything. Course if that is true, then it can't have a speed.

Sometimes the definition of the problem precludes you from finding the answer. In Galileo’s time the definition of astronomy was "the study of how thing move around the earth." Preventing the discovery that things don't move around the earth.

That is what we have on this thread, they don't realize that they are standing between the plates of a huge capacitor and connecting the parts together, no matter how elaborate, will not cause electricity to flow. You have to connect the parts outside the capacitor to get that to happen.

 

[T37 Phil]

A hydrogen atom (one proton and one electron) is about a centimeter in diameter out in space. As you bring more and more matter around it it shrinks to the very small size we "normally" see.

Why is that? The electric field is the same in both cases, right?

I have no idea what you are referring to with this one.

As for the previous one, there is still a confusion, perhaps. Imagine a complete and utter vaccum for as far as you need to imagine it. Now put one dimensionless point charge, never mind what it is. It's a point that has the property of charge. Around that point is a a field. What's the field? Excellent question for which I still have no answer. Practically speaking it's a mathematical description of the result of the presence of the charge. Associated with any electrical field is another abstract description called potential. Potential, most simply, can be understood by putting a second point charge into the picture described above. The force exerted between the two particles depends on the polarity and magnitude of the charges, and the position of each with the field. But wait, that's already too complicated. The presence of the second charge actually changes the field in the picture. So take the second charge back out. In fact, take the first charge back out but remember the image of the field that surrounded it. Now pick a location in that field and ask what would be the force exerted on another charge if I put it at that location. That potential force is the electromotive potential associated with that field.

Briefly put, potential isn't a something, it's a measure of the ability to do work. So the 12V measured is a theoretical value that is equal to the force exerted on a unit charges that are a particular distance apart in a particular uniform field of a given magnitude.

Current, on the other hand, is a physical phenomenon. Current is simply the movement of charged particles. Charged particles will move when there is a force on them (electrostatic or otherwise). An electric field (and it's associated potential) will cause a particle movement of charged particles when there are charged particles present and 'free' to move within the field. A conductor plays both of those roles. A conductor provides free electrons that move as a result of the potential that is associated with a particular field. In this thread the issue of 'how' a current flows through a conductor and what the field and potential associated with the conductor are have been mostly skirted, as I will continue to do.

Electricity really seems to have two branches, electronics/circuit technology and the physics of electricity/electric fields. Somehow lightning (and beer?) seems to bring the two together in a flash of noise and heat that is wonderful if you're watching from a distance and perhaps less so if you're sitting under it.

 

[Bill Roosa]

Great disertation Phil

What is your point?

Now imagine that you are standing inside a large capacitor (flat plate with one above and one below). Adjust the voltage on the top plate so that you get 10 volts / foot electric field gradient and ground the bottom plate. Move to the center of the capacitor well away from the edges so we don't get "the outside" interfering with "the inside". My head is at 60 volts and my feet are at 0 volts. If I pull a wire out of my pocket and stand on one end and hold the other end to my head does current flow?

No, because the top of the wire is at 60 volts potential. Now run the wire under my foot along the bottom plate around the edge (and out of the E field) and connect it to ground. I'm dead, why? In both cases the wire has 60 volts potential on it. But in the latter case it has a portion that is not in the E field so the current can flow. Everything in the E field has the same gradient voltage (a scalar property), it only has potential (a vector property) when you compare it to something outside the capacitor. I like to think of it as the wire allows the E field to vent to ground. It cannot vent inside the capacitor for the same reason the water at the bottom of the lake, under high pressure, cannot vent to the surface. There is an E field in the way.

 

[T37 Phil]

What is your point?

No, because the top of the wire is at 60 volts potential. ...
In both cases the wire has 60 volts potential on it.

But in the latter case it has a portion that is not in the E field so the current can flow. Everything in the E field has the same gradient voltage (a scalar property), it only has potential (a vector property) when you compare it to something outside the capacitor. I like to think of it as the wire allows the E field to vent to ground. It cannot vent inside the capacitor for the same reason the water at the bottom of the lake, under high pressure, cannot vent to the surface. There is an E field in the way.

I don't think we're seeing this eye to eye. Forget about the wire for a second. There is a field that exists between the plates. You didn't mention if your feet are on one plate and your head on the other, but no matter. If you take out a glass ruler and hold it horizontally in the field, every point along the ruler will be at the same potential. If you turn it perpendicular to the plates, now the field will change [linearly] from position 0 to position 12". Now, swap the ruler for a conductor and do the same thing. In the first case, there is no mystery. Equipotential, no current flow. What about the second case? The question becomes what is the field 'inside the conductor'. If you say it's the same as the outside, current must flow. If you say it's equipotential, then what's the potential? Walt has said that it will be the average of the potential along the length of the outside of the conductor. I haven't given my final answer yet.

Let me reiterate that I believe you are thinking about this problem in terms of a circuit, ie no current flow until you complete the circuit. I am thinking of it in terms of the physics. Where there is a field and free charge, current will flow.

 

[walt]

Walt has said that it will be the average of the potential along the length of the outside of the conductor. I haven't given my final answer yet.

Yup, still say that. If you "buy" that an insulated conductor takes on a single voltage in an electric field equal to the field at its geometric mid point (assuming orientations are all correct), then this also says that the electric fields at both ends of the conductor will be concentrated (ie, higher gradient) compared to the free space electric field.

Here is a little stretch.. but I think this same idea would apply to a lightning bolt - ie, a propagating bolt is somewhat of a conductor with (I believe) the resulting highest electric field gradient at the propagating edge.

Something to ponder also while you have the electric field experiments going on inside your head... (quote from a "winter" discussion last January)

The speed of lightning in one reference is 130K miles per hour (or 36 miles/sec). While fast, it’s a huge amount slower than the speed of light.

At the ends of a propagating lightning bolt is a very intense electric field. The field “effects” of the bolt travel at the speed of light and can “charge” conductors such as mast and create electric field disturbances a long distance ahead of the propagating leading edge of the bolt.

In turn, the Electric field disturbances at some distance away propagate back up to the lightning bolt also at the speed of light. Charge is bound by ohms law so to some extent, lightning has an “eye” and can see in advance of where it wants to go.

(fortunately, lightning apparently doesn't care that much about "minor stuff" like a mast)

 

[Bill Roosa]

Glass rulers

No Walt
the glass rule will be a different potentials just like everything else inside the field. That is the point you and everything is in a uniform field. The thing that makes the capacitor act like a capacitor is the external field. that is what allows the current to flow. Remember no current flows between the plates of a cap. On earth we do not have access to the external field as that is above the ionosphere.
If you have a metal rod inside the capacitor and try to get current to flow how are you going to do it with conductors returning the electrons to the top of the metal rod? You can't because those electrons would have to fight their way up the electric field. You HAVE to go outside the field to get a vector potential. Everything insdie the field is a scalor.
In any case this E field is not what makes lightning. Lightning is superimposed over the existng static E field.

 

[Bill Roosa]

Current flow in something not a circuit

Well Walt I agree that if there is a force on an electron something happens. If it is not a circuit that you have to consider the capacitance of the parts. So in my big cap experiment charge flows to my head till my head is at 60 volts potential exactly canceling the 60 volt E-field.
And we have come full circle with my "charged up boat" hypothesis.

 

[T37 Phil]

Yup, still say that. If you "buy" that an insulated conductor takes on a single voltage in an electric field equal to the field at its geometric mid point

A mast is not an insulated conductor.

this also says that the electric fields at both ends of the conductor will be concentrated (ie, higher gradient) compared to the free space electric field.

The presence of the conductor does not influence the field at all, unless it is either charged in a way that is not 'neutral', or geometrically arranged such that charge is distributed because of the field, resulting in additional field sources.

The quote from last year seems fuzzy headed. The field is not only at the ends of the bolt. The visible bolt image travels to a viewer at light speed. It is the current flow that causes the heating that causes the glowing that is eventually seen by an observer, that travels more slowly. I must be missing why there is anything else unusual going on there.

 

[T37 Phil]

Well Walt I agree that if there is a force on an electron something happens. If it is not a circuit that you have to consider the capacitance of the parts. So in my big cap experiment charge flows to my head till my head is at 60 volts potential exactly canceling the 60 volt E-field.
And we have come full circle with my "charged up boat" hypothesis.

Whatever block box 'circuits' you consider attached to the plates of a capacitor, they either add or remove charge from the plates. The difference in charge across the plates is what constitutes or defines a field through the dielectric (and it's associated potential). A real (non-ideal) conductor, placed in the dielectric of a capacitor normal to the plates (but not touching them), will either experience a current within itself until the field within the conductor cancels out the field from the plates and the situation becomes static again (and the field in the dielectric distorted in the neighborhood of the conductor), or the inside of the conductor does not 'see' the field between the plates at all, in which case no current flows (as one versed in circuits might expect). I'm leaning towards the latter, but I haven't confirmed it for myself yet.

 

[T37 Phil]

The thing that makes the capacitor act like a capacitor is the external field. that is what allows the current to flow. Remember no current flows between the plates of a cap.

See my last post. External field and capacitor (a circuit device) don't really go together. There is an external current that supplies charge to the plates, resulting in the field/potential, whichever you care to consider.

On earth we do not have access to the external field as that is above the ionosphere.

Huh?

If you have a metal rod inside the capacitor and try to get current to flow how are you going to do it with conductors returning the electrons to the top of the metal rod?

Again, this is a notion from wire and component circuit electronics.

Everything insdie the field is a scalor.

You've said this a few times. What do you mean by it? Every point in an electric field is characterized by a scalar, that being the magnitude of the field at that point. If you consider field lines (related to flux), they might be described by a set of unit tangent (or normal) vectors, but that's strictly direction info. If you consider the force exerted on a charge at a location in the field, that force has magnitude and direction and would be an element in the actual 'vector field' that is the electric field.